1. Field of the Invention
The present invention relates to a power module, and more particularly, to a power module for improving a substrate characteristic and a low thermal resistance characteristic.
2. Description of the Related Art
When isolated power modules are fabricated, one or two semiconductor chips are mounted on a lead frame die pad and packaged using a molding resin such as an epoxy molding compound. The epoxy molding compound is used as an isolation material and simultaneously used as a heat transfer path.
To achieve low cost, compactness, lightness, low noise, and high reliability of power conversion systems with high speed, large capacity, and high degree of integration of electronic devices, which are used in automobiles, industrial machinery, home appliance, etc., heat generated in a semiconductor chip should be efficiently discharged.
Accordingly, a direct bonding copper (DBC) substrate or an insulated metal substrate (IMS) is widely used for power modules. The DBC substrate is fabricated by pressing and bonding a copper layer to bottom and top surfaces of a ceramic insulating substrate. The top surface may be partially formed in a designed pattern. The IMS is fabricated by forming a polymer insulating layer on an aluminum substrate and forming a copper layer in a designed pattern on the polymer insulating layer.
FIG. 1A is a cross-sectional view of a power module using a conventional DBC substrate for low thermal resistance. Referring to FIG. 1A, the power module includes a conductive layer adhesive 20 disposed on a DBC substrate 10, a device layer 30 fixed by the conductive layer adhesive 20, and a sealing material 40 hermetically sealing each of devices included in the device layer 30.
As described above, the DBC substrate 10 is fabricated by bonding an upper copper layer 16 and a lower copper layer 12 to top and bottom surfaces, respectively, of a ceramic layer 14. The device layer 30 includes a passive circuit 34 and a power semiconductor device 32. The device layer 30 may also include a pad for connection to a lead line 50.
The sealing material 40 may be usually made using a molding resin, for example, epoxy, in a transfer molding process. A support pin hole 45 is formed in the sealing material 40 by a support pin pressing against the DBC substrate 10 during the transfer molding operation. The support pin is removed after the molding operation. During the molding process the flatness of the DBC substrate 10 is maintained by the support pin pressing against the DBC substrate, and the generation of a mold residue or mold flash is prevented.
However, when the support pin presses the DBC substrate 10, a stress is applied to the DBC substrate 10, which sometimes causes a fault like break of the ceramic layer 14 in the DBC substrate 10.
FIG. 1B is an enlarged perspective view of part A illustrated in FIG. 1A. Referring to FIG. 1B, a support pin 60 is pressing the DBC substrate 10. Thus, a stress induced by a compressive force is applied to the DBC substrate 10. As a result, the ceramic layer 14, which is brittle and has little ductability, sometimes partially cracks or breaks as shown in part B illustrated in FIG. 1B, thereby reducing isolation breakdown voltage. Consequently, the entire power module fails.